Note that the iMac came with 1GB of RAM which I upgraded in January as an attempt to solve the problem.

The problem:

I started having weird graphic bugs after I upgraded to Leopard. Whenever I clicked on a Stack or used Spaces (graphics intensive stuff) some weird lines (purple, cyan, yellow) would remain on screen flickering (drove me mad when watching a movie) or sometimes the whole OS would just freeze and beachball for 10 minutes (or simply going into Kernel Panic).

I figure that the lines are software-based because they do show up when I take a screenshot. I've already went back to Tiger and also found the system very unstable and occasionally it still gives me the colored lines and graphical bugs (and I can't install iLife 09 which sucks).

I just re-installed Leopard last night (from Tiger, again) and with a clean install, after installing all the updates (10.5.8 and etc) the first time I boot into the OS Safari locks up and everything freezes. I had to manually turn off the iMac.

I ran the hardware test last night (extended mode) and it says that all the hardware is fine.

Can anyone point me in any direction? There's no explanation for the system crashing all the time with a fresh install of the most recent Apple OS unless it is a hardware issue.

I'm losing my mind with this, my MacBook is behaving just fine (haven't had one app crash on me with Leopard) but the iMac is seriously giving me headaches...

If you have any suggestions... I'm all ears

Thanks in advance and sorry for the mess I made trying to explain the problem but I did the the best I could.

Yes, I've already went back to Tiger but the system was still unusable.

Beachballed all the time simply browsing folders (especially from external hard drives) or switching between Firefox, iPhoto and Mail.app.

I just don't know what else to do especially because in my country (Portugal) there isn't one single official Apple Store, only authorized resellers.

Like I said, I just installed Leopard yesterday and I've already seen some bugs starting to appear (the software update panel had a line just like I described above). I'm afraid something is wrong with the GPU but isn't the Hardware Test suppose to catch that kinda stuff?

sounds like an occasional problem i have with my 2007 imac, where it shows (what looks like) old screen images over the current image, twisted and rendered in some weird way, with different colors. I thought it might be a vram problem. Has anyone else had this?

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